TIN-BEARING PEGMATITES OF ERONGO AREA, S.-W. AFRICA.
THE
TIN-BEAIUNG
PEGMATITES
SOUTH - \V EST
OF
THE
EHONGO
111
AREA,
AFRICA.
(Read 29th October, 1929.)
By T. W. Gevers, M.A., D.Phil., and
H. F. Frommurze, B.Sc.
(Published by authority of the South-West African Administration.)
[PLATE
VI.]
INTRODU CTION.
In South-West Africa an extensive tract of country situated in
the north-western portion of Damaraland and of which the E,rongo
mountains form the dominant morphological fea,ture is generally
referred to as the " Erongo Tinfields. " Cassiterite was first discovered
here in the north-western portion of the Omaruru division in the year
1909. In the following year considerable deposits, mainly of an eluvial
nature, were found on the farm Ameib, about 18 miles north-west of
Karibib, and situa,ted immediately below the imposing southern escarpment of the ErongOi mountains amid an unrivalled wealth of
magnificent granite scenery. Owing to the promising nature of these
fields, extensiv~ prospecting operations were carried on, and to-day the
tin-bearing area has been shown to extend from south-east of Karibib
to beyond the Brandberg and Ugab as far as the southern portion of
the Koakoveld, a distanc~ of approximately 140 miles. The exact
limits of the fields have, however, not yet been defined.
Within the area so far investigated the tin-bea,ring pegmatites are
found to occur along three well-defined main belts. The orientation of
these belts is determined by the general and local strike of the ancient
sediments and the contacts of the intruded old granites. Beginning
in the north-west what may be referred to as the Northern Belt begins
with profuse development of pegmatites around Dis, and from here
continues westwards over a distance of about 25 miles to Ploeger's
claims south of the Brandberg. The exact distance of its further
extension westwards tOlwards the sea is unknown.
The Great Central Belt along the Omaruru River commences on
the lower course of this river west of Neineis and then continues over
Nobgams, Humdegams, Tsomtsaub, Paukwab, the Thelma Mine and
Kohero as far east as the western slopes of the Kompaneno mountains,
north-west of Omaruru township, a total distance of about 60 miles.
This belt comprises some of the most important tin-occurrences of the
whole fields.
The Great Southern Belt commences in the west a, short distance
north of Ebony, and then continues past Sandamab, between Usakos
and the Grea,t Spitzkopje, to the south-western corner of the Erongo,
fringes the foot of its southern escarpment, crosses the Khan River a,
few miles north of Karibib, and then continues along the banks of this
112
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
rIver a.s a, continuous belt a.s far as Otjimboyo and Otjakatjongo, 30
miles east of Karibib. The total length of this belt is approximately
80 miles. Near the so.uth-western corner of the Eron.go this great belt
sends off a subsidiary branch along the slopes of the western escarpment Otf these mountains, while another subsidiary belt lea,ves the
main belt near the confluence of the Khan and Etiro rivers and undergoes its main development on Erongo-Kanona below the T'.P. Erongo.
It was the investigation of these tin-deposits, from which during
the past ten years there have been produced 2,100 tons of concentrates
of an average content of over 70 per cent. Sn0 2 , which formed the
main objective of the geological survey of this area,. The following
paper is mainly intended a.s a, brief summary of the results obtained
and the conclusions arrived at.
Before entering on the discussion of the tin-bea,ring pegmatites a,
short table of the general geology of the area will be given in order to
show the relationship of the pegmatites to the grea.t vaJ'iety of intruded
granites. For further details concerning the general geology reference
should be made to the previous paper on this area. l
TABLE OF THE GENERAl.J GEOLOGY.
Sediments: 'Vindblown sand.
Rock debris a,ccumulated unde.r arid conditions. Gra,vel and
{ boulder terraces of the Namib Kalahari
Limestone.
Sediments
of
the}
Erongo, Brandberg Shales and mudstones.
Felspathic sandand 0 t jon gun d u
stones. Arkoses, grits and conglomerates.
Plateau
Surface Deposits
Schist Formation
{Mica, schists. :Marbles and amphibolites.
Qua,rtzites and mica schists.
Igneous Rocks.
The Erongo Granite.
Porphyry tuff. Dykes and flows of acid porphyrites and qua,rtz porphyry. Dykes of
diorite porphyrite and granite porphyry.
Igneous rocks of the
Stocks of diorite and granodiorite. Basic
Eron.go Succession
tuffs. Dykes and flows of augite porphyrite. Dykes and flows of melaphyre,
frequently amygdaloidal. Dykes of ophitic
diabase, and dolerite.
Pegmatitic granite. Aplitic granite. NonOld Granites
porphyritic Salem granite. Red Salem
{ granite. Salem granite. Gneiss-granite,
1 "The Geology. of N orth-Western Damaraland in South-West Africa,"
Gevers and Frommurze, Trans. Geol. Soc. S.A., Vol. XXXII, 1929.
TIN-BEARING PEGMATITES OF ERONGO AREA, S.-W. AFRICA.
113
TECTONICS.
The ancient sediments of the Schist Formation and of the Fundamental Complex in general have everywhere been intensely folded. rThe
general strike of the sediments is N.E.-S.\V., but this grea,tly varies
locally. 1'he folding has taken place on very complex lines and exhibits
the powerful influence of multilateral stresses. Dome-like structures
are as a result very common. The southern limbs of structural elements
almost invariably possess a, steeper dip than the northern. Actual overfolding is very common and invariably directed from the south-oast or
south to the north-west or north. 'rhe main lateral stress therefore
came from the south-east and south.
The intrusion of the old granites has generally taken place
on conformable lines. The contacts are predominantly concordant and
the bodies of granite in the area, under discussion generally phacolithic
in na,ture. Very frequently the granites fill synclinal structures or
form the cores of anticlines and dome-structures. The strips of sedi~
ments, usually schists sepa,rating granite massifs, are frequently very
narrow, and their strike determines the strike and alignment of the
various tin-bearing belts.
From this it would appea,r tha,t the vast crustal invasion which
began with the intrusion of the Salem granite, and culminated in the
injection of innumerable pegmatites, took place while the folding
movements were still in active progress.
There have been at least
two periods of folding: one still adive while the old granites and
associated pegmatites were intruded, and a later upheaval subsequent
to their intrusion, and to the deposition of the sediments of the
unconformably overlying Phyllite formation. A long period of time
therefore separates these two periods of intense folding.
The very much more youthful sediments 0'£ the Erongo, Brandberg, Otjongundu Plateau, etc., and the Wa,terberg sediments of
Upper Karmo age, with which the former sediments a,re mnst probably
approximately contemporaneous, ha,ve not suffered any appreciable
folding beyond faulting and warpiD,g on a considerable scale.
THE PEGMATITES.
A great variety of different pegmatites is met with in this part of
South-vVest Africa. 2
The bulk of these may be divided into two groups: Tou,nnaline
.
pegma.tites and tin-beaTing pegmatites.
Mode of Occunence and DistTibu,tion of the Pegmatites.-As has
?een stated already, the old granites and associated pegmatites were
mtruded while the folding of the ancient sediments was in active
• 2 Reuning,
E.: "Pegmatite und Pegmatitmineralien
Zettschr. jiir J(rystallogr., etc., Vol. LVIII, 1924.
III
S.-W.A.,"
114
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
progress. It is also to be expected therefore that. the pegmatites
exhibit a very definite rela,tionship to the structural plan of the
sediments.
While there is a grea,t number of pegmatites in the parent granite
bodies themselves, pa,rticularly in their marginal portions, they attain
their greatest development in the enveloping schists. Usually they
are most abundant in the neighbourhood of the granite contact. It is
a noteworthy fact that pegmatites are rare in the crystalline limestones
so abundantly distributed over the whole tin-bearing area" even .when
these are in direct contaet with the parent granite. The latter also
very seldom break through these apparent,ly highly rigid rocks. Within
the schists they occur in distinct groups, and the presence of these
can usually be ascribed to one or more structural features.
Numerous bodies of granite, pha,colithic in nature, occupy positions
marked by a very powerful torsion of the schists, the space occupied by
the granite having mainly been formed by differential movements along
~he bedding planes of the sediments. The contact in the inner arc of
compression is frequently chara.cterized by a crumpling of the sediments and generally an absence of pegmatites, while the outer arc of
tension is very generally accompanied by extensive disruption of the
sediments (schists) and a consequent abundance of oriented xenoliths
in the marginal portions of the granite. In addition there is generaJly
also a profusion of pegmatites in the region of the outer arc. This
feature is very general and grouping of pegmatites is more
apparent where the schists ~xhibit bends.
Differential movements along the bedding planes in zones of tension evidently
greatly aided the magma in its upward ascent. "Where the schists
straighten out again, the pegmatites usually become much less
abundant and even completely disappear. An intricate net-work of
dykes is almost invariably connected with bending movements in the
schist frame.
Other factors favouring the intrusion of dykes a,re connected with
the close neighbourhood of two or more granite-bodies separated by a,
schist fram~ of only narrow width. Such a, strip of schists is then
usually intensely invaded by dykes. Also where such a, strip of schists
is suddenly cut off across its strike by an occasional discordant granite
contact, the number of dykes increases considerably. In the latter
case all the bedding planes and planes of schistosity lay directly open
for attack by the magma, and the ease with which the latter could
effect penetra,tion of the schists was as a result materially enhanced.
It is also a· noticeable fact that pegmatite dyl):es are usua.lly very
much more abundant at localities where the dip of the schists is st 3ep
than at places where this is low, as, for instance, the curving terminal
portions of pitching anticlines and domes. Evidently the, great weight
of the overlying rocks rendered penetra,tion by the magma very much
more difficult where the dips were low than in cases where penetration
merely enta,iled the lateral forcing apart of sediments, particularly
l
TIN-BEARING PEGMATITES OF ERONGO AREA, i3.-W. AFRICA.
115
since in its effort to effect this the magma was greatly aided by
differential movements along the steeper bedding planes.
By far the greatest number of dykes are conformable strike-dykes,
i. e., paJ"a.llel to the strike and dip of the schists. In the area
immediately south-west and west of the E:rongo of the 1,710 pegmatite
dykes that were noted and of which directions of strike were measured,
7H per cent. were oriented parallel, 13 per cent. at right angles, and 8
per cent,. at diagonal and oblique angles to the, strike of the schists.
While the majority of dykes, whether strike, cross or diagonal
dykes, in anyone area show no differentiation with regard to their
respective ages, it is a noteworthy fact, tha,t dykes obviously somewhat
later and cutting pa,rallel dykes are usually cross or oblique dykes.
This fa,ct indicates that after the first injections the sedimentary frame
had already become: somewhat consolidated by the folding and presence
of the congealed granites and pegmatites and that the lines of smallest
resistance and easiest penetration had already been occupied. The later
magma was therefore obliged to follow joints and fissures across the
strike of the sediments.
The thickness of the pegmatite dykes va,ries from a few inches to
50 feet and more. Their thickness usually decreases with the distance
from the granite-contact. The height wit·hin the schist fraJ.ne, to which
the pegmatites have been intruded naturally varies greatly.
The shape and behaviour of the individual dykes also vary greatly;
some are folded, others undulate in the horizontal plane and jump from
bedding plane to bedding plane, both horizontally and vertically.
Others possess numerous constrictions and exhibit the pinch-and-swell
structure.
The Relationship of Pegma,tites and Aplites.-Heldring, in his
article on the Tin-Fields of South-West Africa, makes the statement,
"that the pegmatite changes into aplite, and this again into quartz
with increasing vertical or lateral distance from the ejecting centre. 3
Where true pegmatite-reefs cut quartz-reefs, there is no doubt that the
former have originated from a higher horizon than the latter." The
available field-evidence, however, does not suppo,rt this view. On the
contrary, pegma,tites, aplites and bodies of pegmatitic quartz have
been found occurring together in the same locality, both near and
further away from granite-contacts and giving no indication of a ,gradual
transition from pegma,tite over aplite to quartz. In these cases no
assumptions with regard to theoretical horizons and distance from the
ejecting centre could account for their irregular and closely neighbouring occurrence. In addition, numerous bodies of pegmatitic quartz
were found to be intrusive into ordinary pegma.tite and to belong to a
~efinitely later period of intrusion. Finally, wherever observed, as, for
mstance, on Paukwab and Erongo-Kanona, and numerous other locali~ies, . the aplites, frequently present in great abundance, were
mva,rlably found to be cut and faulted by the pegmatites and, therefore, to belong to an earlier phase of injection.
8 Reldring, O.
G.: "Some Notes on the Tin:fields of S.W.A.," Mining
Journal, Vol. OLXI, Nos. 4837-4841.
116
'fRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
It must be concluded, therefore, that in our case the aplites
generally belong to an earlier phase of magmatic diff~rentiation
preceding the injection of typical pegmatites.. They consls.t almo~t
entirely of felspar and quartz, a large proportIOn of these mmerals m
gra,phic intergrowth, and pneumatolytic activity is generally still .very
restricted, though black iron- tourmaline, muscovite, and e·ven occasIOnal
gra,ins of c,assiterite are found in them.
These features, coupled
with the fact that pegma,titic quartz was very generally intrusive and
therefore later than the ordinary pegmatite, and the results obtained
by Wright and Larsen 4 with regard to the different temperatures of
crystallization 0'£ the quartz found in pegmatites, suggest that the
theory of eutectic residues, mainly advocated by Vogt, possesses in
many instances only a restricted application.
TOURMALINE-PEGMATITES.
The basis of these and the tin-bearing pegmatites is a coarsegrained pegmatitic rock consisting almost entirely of felspa,r (orthoclase,
microcline, albite and perthitic intergrowths of albite and orthoclase) and
quartz, a large proportion of these minerals usually in graphic intergrowth. The felspar when fresh is .generally of a pure white colour,
pe.gma,tites of this type possessing red felspars being very subordinate.
Black iron-tourmaline (schod), frequently in large well-formed
crystals, is the characteristic mineral of the non-tin-bearing varieties of
the pegmatites. Almost invariably it is found to be a product of
replacement both of the felspar and quartz present. Tourmalinecrystals containing an irregular central core of unreplaced quartz are
common.
The replacement of thE} felspars by tourmaline usually follows the
cleavage of the felspars. Very frequently it is almost complete, particularly along joint-fissures in granite and pegmatite, with the production
o,f an intimate intergrowth of quartz and black tourmaline. Occasionally distributed throughout the whole dyke-rock, it usually is more
abundant in and even restricted to distinct bands of repla.cement, very
commonly in the. marginal portions of the dykes. Along the walls of
the dykes it is common for the crystals to be arranged with their axis
of elongation a,t right angles to the walls. If muscovite accompanies
the tourma1ine, it is seldom in large quantities.
At Paukwab,
tourmaline-bearing pegmatites occasionally carry biotite.
At this
locality bla.ck tourmaline is also directly associated with cassiterite.
Tourmaline pegmatites are most numerous in the -parent granite
and in the neighbourhood of the granite conta,cts, where they frequently
occupy a definite zone of varying width. Further away from the contacts the tourmaline (schorl) is often almost entirely replaced bv
muscovite and cassiterite.
L
4 Wright
and Larsf'n: "Quartz as a Geologic Thermometer," Amer.
JOtl.rn. Science, Vol. XXVII, No. 162, 1909.
TIN-BEARING PEGMATITES OF ERONGO AREA, S.- W. AFRICA.
117
GARNETIFEROUS PEGMATITES.
Garnets, usually of the composition of almandine, frequently
grading into spessartite, are a common constituent of many pegmatites
and pegmatitic granites. Very commonly the garnet is associated with
schorl, and the a,vailable evidence indicates that it is pneuma,tolytic in
nature and similar to black-iron tourmaline in ,general deposited at
higher temperatures than most of the other pneumatolytic minerals.
r:ehe occasional association of these iron and manganese garnets with
triplite, as, for instance, on Elliot's claims below the Hohenstein, is
of special interest in this connection.
THE T'IN-BEARING PEGMATITES.
All the tin-bearing pegmatites exhibit as their basis the same
gangue-minerals as the tourmaline pegmatites: orthoclase·, microcline,
primary plagioclase (albite), often m micro-perthitic intergrowth with
orthoclase, and quartz, a large proportion 0& the latter usually in
graphic intergrowth with the felspars. :Most pegma.tites also contain
considerable quantities of secondary qua,rtz, particularly those
appreciably altered by pneumatolytic action.
The grain of the rock va,ries ,greatly, no doubt partly as the result
of pneuma,tolytic alteration, particularly albitisation. Usually coarsegrained, the pegmatitic rock also assumes a, fine-grained habit and
becomes aplitic in appearance. It is common to find a composite dyke
consisting of a coarse-grained and fine-grained a,plitic portion, both tinbearing and sometimes separated by a narrow schist parting. Very
commonly the typical rock consists of coarse portions characterized by
large fE)lspar crystals separated by a fine-grained mesostasis, of which
graphic intergrowths of felspar and qua,rtz form a, considerable proportion. This. portion of the rock evidently was the last tal crystallize.
The mode of occurrence of the cassiterite, as well as its mineral
associations and the general habit of the tin-bearing pe,gmatites, vary
to a considerable extent, and several types of tin-occurrences were
observed associated with the pegmatites.
MODE OF OCCURRENCE OF THE CASSITERITE.
The cassiterite is generally distributed in definite zones and bands
of repla,cement, these having a, definite relationship to the contours
~ walls) ?'f the dykes or the intrusive bodies of pegmatitic quartz, but
IS occasIOnally scattered throughout the whole rock as fine tin. This
~s m07'8 often the case in the finer-grained aplitic-looking pegmatites,
III w.hlCh very fr~quently. the cassiterite is unaccompanied or only very
sparmgly assocIated WIth other pneumatolytic minerals such as
tourmaline or muscovite, though a widespread albitization of the
primary potassium-felspars is noticeable.
118
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
The ca.ssiterite occurs in well-formed crystals, irregular grains, and
also in the form of large lumps, which frequently exhibit a few
crystalline faces. Some of these lumps attain extraordinary dimensions, as may be judged from the fact that at Dawib a homogeneous
mass of cassiterite weighing 500 lbs. was found in the early da,ys of
the fields. Large lumps and crystalline aggregates are usually found
in association with an intense greisenization of the original pegmatite,
the size of the lumps usually being directly proportional to the, dimensions 0'£ the muscovite and the intensity of greisenization.
The
crystals or cassiterite, which va,ry in size from microscopic dimensions
to a diameter of several inches, are commonly twinned and in consequence exhibit characteristic re-entrant angle~. The habit of the
crystals is predominantly pyra,midal, the prism faces being as a rule
very poorly developed. In thin sections the mineral very frequently
exhibits a zonal structure, the crystals showing symmetrical or irregular
patches of darker and lighter colour, brown and yellow.
The colour of the mineral also varies considerably, the prevailing
tints, however, being yellowish-brown and brown.
Occasionally,
however, the colour is almost black, as, for instance, at Paukwab.
Here the mineral occurs intimately intergrown with black irontourmaline and, similar to this minera-l, it is also minutely brecciated.
As a- result it is very difficult to distinguish the two minerals,
macroscopically. Frequently a reddish streak is the only indication of
the presence of cassiterite in a- mass apparently consisting entirely of
black tourmaline.
At Otjimboyo and a few other localities true ruby tin is found.
Here also large well-formed crystals of a beautiful resinous-yellowishbrown, transparent va,riety of cassiterite were taken from drusy cavities
in the schists.
An interesting sample which was obtained shows the bulk of the
crystalline cassiterite to consist otf a whitish-grey variety but displaying
a number of zones, pa,rallel to pyramid fa.ces, formed by dark-brown
and light-yellowish-brown varieties in successive stages of growth of
the crystalline mass.
DIFFERENT TYPES OF TIN-OCCURRENCES.
With the exception 0'£ a few isolated tin-occurrences not connected
with pegmatites and to be discussed subsequently, the cassiterite is
found to be associated with pneumatolytically altered pegmatites.
There are, however, a number of types of tin-bearing pe,gmatites, each
with different mineral associations and exhibiting distinct characteristics.
Two main divisions ma,y be made:A.
B.
Tin-bea,ring pegmatites situated in the parent granite.
Tin-bearing pegmatites situated in the adjoining schists.
The latter group may be further sub-divided into:-
TIN-BEARING PEGMATITES OF ERONGO AREA, S.- W. AFRICA.
119
(a) Pegmatites containing cassiterite unaccompanied by
tourmalinization and greisenization on any extensive scale.
(b) Ordinary, ,common type of tin-bearing pegmatite containing
cassiterite accompanied by muscovite and, to a lesser
extent, bla.ck tourmaline.
(c) Intensely ,greisenized pegmatite associated with intrusive
bodies of pegmatitic quartz.
C. Cassiterite deposited from hi,ghly silicic aqueO'us solution and
not associated with pegmatites.
A. Tin-Bearing Pegmatites situated in the Parent Granite.Compared with the number of tin-bearing pegmatites occurring in
schists, these are comparatively limited in number. At Paukwab and in
the Erongo-Kanona area" are to be found the most important examples.
At Carsie, where the tin-bearing pegmatite is situated in a very small
extension of the main granite mass and thus not really in the parent
body, it shows features comparable to the schist-type of tin-bearing
pegmatites.
On the whole these dykes a,re not as \vide as the majority of dykes
occurring in schists. Many of them are mere narrow and meandering
veins.
At Paukwab, the bulk of them occupy joint directions parallel to
the general strike of the schist (N.E.-S.vV.) and not to the local, ver~
much disturbed strike of these sediments. It is interesting to note,
however, that the abundant dykes of aplite present conform to the
local directions of strike and stress and, incidentally, are invariably cut
and even faulted by, and therefore older than the pe,gmatites. (Fig. 1.)
The pegmatite dykes are coarse to fine-grained and sometimes
contain both varieties in the same, dyke. T'hey are characterized by
the abundance of black iron-tourmaline (schorl) and the intimate
association of cassiterite with this mineral. Both associated and
intergrown minerals generally occur in definite bands and zones of
replacement throughout the pegmatitic rock and are usually most
abundant along the walls of the dyke. Muscoyite is almost entirely,
absent, but biotite is occasionally develo,ped.
In addition, garnet
(almandine-spessartite) and topaz are associated with cassiterite and
schor! in some zones of repla,cement. At the points of intersection of
the earlier aplites with the la,ter pegmatites, the rock of the former
is sometimes also minera.lized to some extent.
An interesting point is, that wherever the granite-schist contact
is exposed, these pe,gmatites do not continue into the schists. On the
contrary, a dyke intruded along or near the actual contact, the so-called
" contact-reef" is of characteristic schist-type with an abundance of
muscovite. Further, all the pegma,tites occuning west of the diabaseridge, separating the granite from the schist area, although occurring
in the same o'r even at a lower level, are t'ypical schist-pegmatites
characterized by the mineral association: muscovite-cassiterite. The
] 20
TRANSACTIONS OF THE GEOLOGICAL SOCIE1'Y OF SOUTH AFRICA.
5k.e tc.1t rIo. n of a/:;tri 6uilOn ~ ori, n tD.t'Olr oj Jy ke~
on Po. II It W o..lr.
eJ1n a/tre s
@ij AJma Tire s
e'4 QAli [{?s fa.u Iteel &y
E3 9/,La..,.tz V(/nS
E3 l1/a 6a~e. d.ylie
m Schists
t
F'IG. 1.
TIN-BEARING PEGMATrfES OF ERONGO AREA, S.-W. AFRICA.
121
change from one type to the other is therefore very abrupt here at
Paukwab and takes place with the change of country rock. At ErongoKanona, similar conditions hold, only that, here a, pegmatite dyke
situated in Salem granite, but only a fe.w yards away from the schistgranite contact, is of the schist-type.
'The Salem ,granite at Paukwab is intruded more or less conformably into a synclinal structure of the se.diments and the pegmatites
occurring in the adjacent schists cannolt, be considered to be continuations of those occurring a,t, the same level in granite. All the available
evidence suggests that the site of actual intrusion and passage from
the granite to the schists in the case of the former lies very much lower
than the present surface and the level of the pegma,tites exposed in the
neighbouring granite. 'rhe schist pegma,tites were evidently intruded
from deeper-seated residual magma-portions at a peried somewhat later
than those pegmatites outcropping in the granite at the sa,me level.
The continuation of these latter pegmatites into schists must be looked
for in higher horizons of the schist, now removed by denudation. This
sequence, deduced from structural considera,tions, a.lso accounts for the
abrupt change in habit and mineral associations of the two series of
pegmatites and is, moreover, corrohorated by other evidence to be
discussed later, i.e., the deposition of black tourmaline and cassiterite
when associated with this mineral at higher temperatures than
muscovite.
M etasomcdic Processes Involved.-It is quite clear from microscopic sections that both black tourmaline and cassiterite a,re secondary
pneuma,toolytic minerals and the products of metasomatic replacement
of felspar and quartz by gaseous solutions containing boron and tin.
These pneuma,tolytic changes were., a,ccompanied by the albitization of
orthoclase and particularly microeline on an extensive scaIe.
The
black tourmaline replaces both the primary felspar and the secondary
albite, as well as primary quartz. The. cassiterite also' replaces both
felspars and quartz, but shows a marked preference for felspar. ~rhe
topaz present, no doubt, was also derived from the decom,position of
the felspars.
Occasionally tourmaline is seen moulded on cassiterite, but on
the other hand, according to a statement made by the mine manager,
Mr. Da.vies, a crystal of cassiterite pseudomorph after tourmaline has
been found at Paukwab.
The very intima,te association of the two
minerals suggests that there was no appreciable interva.l of time
between their respective deposition and that they were deposited more
or less contemporaneously. It naturally depends on the concentration
of their constituents in the gaseous solutions to determine which
mineral would continue to be deposited after the vapours had become
completely exhausted of the constituents of one of the minerals.
A further interesting mode of deposition of cassiterite a,t Paukwab
is the zonal distribution in small bipyramidal crystal-grains in large
uniform crystals of microcline, the bands of cassiterite being parallel to
122
TRANSACTIONS OF '1 H1~ GEOLOGICAL SOCIETY OF SOUTH AFRICA.
the zonal growth and cleavage planes of the felspar crystal. This was
found to be of secondary origin. While the majority of cassiterite
grains are euhedral and in the form of bipyramidal ,crystals, quite a,
fair percentage exhibit serrated edges and remnants of unreplaced
felspar.
In addition, bipyramidal crystals of cassiterite frequently
show a narrow rim of later cassiterite, i.e., an enlargement of the
original crystal, which obviously replaces the microcline. rThe uniform
crystal of microcline also encloses numerous small crystals and ,grains
of orthoclase, primary plagioclase and quartz as ea,rlier products of
crystallization.
rrhese ea,rlier minerals are frequently arranged in
bands, i.e., exhibit flow structure within the crystal of microcline. The
cassiterite replaces both generations of felspar and encloses remnants
of unreplaced quartz.
Around some of the cassiterite ,grains the
microcline is turbid and in the incipient stage of albitiza,tion. The
crystals of cassiterite ar'e most abundant along the bands of the earlier
formed primary minerals, showing that the active vapours appa,rently
penetrated the crystal of microcline most easily along lines od: inhomogeneity. Finally, the crystal of microcline also shows a, few minute
clefts parallel to and in close proximity to bands of cassiterite crystals
and along which alteration of the microdine with the production of
secondary albite and secondary quartz has taken place. There is also
no difference whatsoever between the successive zones of growth of the
microcline and no evidence of a break in the growth.
It appears, therefore, that the cassiterite has replaced the felspar
(first generation of orthoclase and plagioclase and the later microcIine)
along minute clefts and submicroscopic channels following the
cleavage planes and successive zones of growth of the microcline and
lines of inhomogeneity within the lafter. The hi,tSh pressure active on
the gaseous solution accounts for the considerable quantities of
cassiterite introduced along such very minute channels.
B.
Tin-Bearing Pegmatites situated in Schists.
(a) Cassiterite una,ccompanied by tourmalinization and gT'eisenization on, an extensive scale.-Occurrences of this type form, only a, subordinate portion of the pegmatites situated in schists. They are
characterized by the compa,rative absence, frequently almost complete,
of muscovite. Black tourmaline is generally more abundant, but on
the whole sparingly distributed. The rock consists practically entirely
of a· white felspar (orthoclase, microcline, primary plagioclase and
micro-perthite), prima,ry quartz, secondary albite and cassiterite,
usually disseminated as fine tin.
The dyke-rock is generally m,ther fine-grained and aplitic. This
is probably to some extent a primary feature, but the Widespread
albitization of the coarse potassium felspa,r crystals no doubt considerably added to it. The unaltered-felspars are frequently somewhat
turbid and clouded.
Very commonly the cassiterite is practically
limited to the fine-grained portions of the rock, thus suggesting a
TIN-BEARING PEGMATITES OF ERONGO AREA, S.-W. AFRICA.
123
connection between the a.1bitiza-tion and the introduction of the
cassiterite. On Otjimboyo 2, east of the" Schlangenrivier," a trench
exposes a dyke about 10 feet in width and consisting mainly of a
coarse pegmatite with numerous closely spaced bands at! replacement
composed ma-inly of muscovite and quartz. The foot of the dyke is
particularly altered in this way (Fig. 2). Unlike the behaviour of
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124
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
cassiterite in the ordinary type of schist-pegmatite, thii:3 mineral is very
rare in these bands of greisen, but it mainly occurs in a fine-grained,
aplitic portion of the rock as abundant fine tin and in the form of
numerous closely spa,ced bands parallel to the, walls of the dyke. The
aplitic portion of the dyke here, however, shows numerous flakes of
muscovite. In the coarse pegmatitic rock cassiterite is not abundant.
Albitization is very much mOire pronounced in the fine-grained, highly
stanniferous portions than in the coarse sections of the dyke. There
is evidence suggesting that the albitization and deposition of
cassiterite preceded the greisenization of this dyke.
A notable feature of the dykes ·consisting of similar rock in this
locality is the highly decomposed nature of the fine-grained rock and
the presence of abundant iron oxides. Not only is the rock here very
much stained by the latter, but the weathering of the rock has also
produced numerous nodules very rich in tin. The weight of such
nodules is often very considerable, and it appears that large and smaIl
lumps of cassiterite or patehes particularly studded with fine tin acted
as inoculation centres around which iron oxide and hydroxide were
deposited during weathering or the la,ter hydrothermal phase to form
stanniferous nodules.
Metasomatic processes involved.-It is this and the previously
discussed type of tin-bearing rocks, in which previous authors considered the cassiterite to be a primary constituent. There are, however, even macroscopic features that do not fa,vour this view. The
cassiterite very generally occurs in distinct bands parallel to the waIls
of the dyke. In dykes of low dip the tin-bands are usuallyconcentrated and frequently entirely limited to the hanging portion of the
dyke. Where dykes intersect or are sharply bent along their strike
or dip there is frequently a, marked enrichment of cassiterite. All these
phenomena suggest the activity of metalliferous va,pours ascending the
fissure marked by the dyke. Where their upward ascent was impeded
marked enrichments of cassiterite are found.
The replacement of felspar by cassiterite is frequently seen
macroscopically. Crystals of felspar are in part replaced symmetrically
along theirclea,vage cracks, and in this way pseudomorphs of cassiterite
after felspar result. (Fig. 3.)
Microscopic sections afford complete proof of the secondary nature
of the cassiterite. First of all any flakes of muscovite that do occur
are almost invariably associated with grains of cassiterite. Further,
even euhedral crystals of cassiterite very frequently exhibit remnants
of unreplaced quartz and felspa,r, and sieve-like grains of cassiterite are
the result. (Fig. 4, a, b, c.) The boundaries of the individual grains
of cassiterite are often serrated and were obviously determined by the
contours of the replaced felspar crystals and quartz grains, i.e., rectilinear along cleavage cracks of felspar and highly irregular and serrated,
when in contact with quartz. Narrow lOTI,S' lath-shaped bodies of
cassiterite pseudomorph after felspar laths are common.
TIN-BEAnING PEGMATITES OF EnoNGO AnEA, s.-w. AFmcA.
tI
I~
FIG.3.
(Jassi terite macroscopically replacing
felspar along cleavage.
A1l1eib.
B
FLa. 4.
Cassiterite replacing feJspar and quartz in microscopic sections
125
126
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
The cassiterite is therefore a product of the pneumatolytic
replacement of felspar and quartz, particularly the former. 'This
replacement is accompanied by the extensive albitization of the
potassium-felspRrs.
C.
Ordinary type of Tin-BeaTing Pegmatites in Schist; Association of
Muscovite and CaSSiterite.
This type of tin-bearing pegmatite is by far the most widespread
representative of the pegmatites occurring in schists. Its characteristic
feature is the constant asso·ciation of cassiterite with muscovite and
its lithia- and fkorine-bearing varieties. Sometimes lepidolite is present
in great abundance, occasionally preponderating over muscovite.
The base of these altered pegma,tites is the sam~' as in the
previously discussed cases, namely-white felspar (orthoclase, microcline, prima,ry plagiocla.s~ and microperthite) and quartz. The former,
i.e., the potassium-felspars have generally suffered extensive albitization.
Frequently black iron-tourmaline (schod) is absent, and coloured
lithia-tourmalines are associated with cassiterite and muscovite in this
type, e.g., at Nobgams, Tsomtsaub, Kohero, etc. In other cases black
tourmaline (schod) is abundant. It usuaUy occupies separate bands
and irregular zones of replacement almost devoid of cassiterite and
muscovite, and has therefore been deposited earlier. Where this is
not the case and all three minerals occur together in the same portions
of the pegma.titic rock, the mineral sequence is not quite clear, although
the black tourmaline hardly ever occurs in direet juxtaposition to
cassiterite.
The degree of greisenization and replacement in this type of tinbearing pegma.tite is modera,te, though portions rich in cassiterite
occur. The replacement by muscovite and cassiterite has usually
taken place in distinct bands or irregular zones. r.rhe marginal· purtions
along the walls of the dykes are particula,rIy liable to be altered in this
way. If the dip of the dyke is steep both walls are generally affected
to an equal degree, but in the case of a, low dip the hanging wall
portion is usually altered and mineralized to a grea,ter extent.
Structural features involving a, concentration of the metalliferous
gaseous solutions or the impeding of their upward ascent play an
important role in determining the relative abundance of cassiterite.
The points of intersection of one or more dykes: sections marked by a,
sudden thinning of a· dyke: the saddle portions of folded anticlinal
dykes: the hanging wall portion of synclinal dykes: sudden vertical or
lateral twists of a dyke and the upper portion of dykes suddenly pinching out are generally very heavily mineralized and sometimes represent
the only sections containing pa,yable quantities of cassiterHe.
At the time of mineralization the pegmatites had evidently
completely solidified and the metalliferous vapours were forced to
ascend along narrow fissures marked by the walls of dykes, clefts
l'RANSAC'l'IONS OF THE GEOLOGICAL SOCIETY OF
N
SOUTH AFRICA.
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5.
Showing mode of greisenization in pegmatite on Gantschows claims,
near Tsomtsaub.
Y'a!i5 Q('"O n.
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of
s(A/~1i
TIN-BEARING PEGMATITES OF ERONGO AREA, S.- W. AFRICA.
127
within the pegmatitic rOlck, joints, bedding planes and planes of
schistosity in the schists.
There are severa,l very clear instances of the ascent of pneumatolytic vapours along fissures ma,rked by bedding planes in the schists
which produced altera,tions only where they came in contact with
pegma,tites. Gantschow's claims near Tsomtsaub on the Omaruru river
supply, perhaps, the most interesting example. Here a dyke parallel
to the strike and dip of the schists and ha,ving a strike of about 100 yards
over the whole length is well mineralized, the cassiterite being associated with muscovite. A cross-dyke, however, branching off at right
angles from this main dyke near its eastern termination (Fig. 5),
widening out in several places and then through a succession of bends
turning into the strike of the schists again, is only mineralized at
intervals, its general tin-content being very poor. At various intervals,
indicated on Fi.g. 5, this dyke contains num~rous bands of greisen
running at right angles through the dyke, but parallel to the strike of
the schist, thus continuing their bedding planes through the
pegmatite. The metalliferous vapours responsible for the greisenization
and deposition of the tin therefore appear to have ascended along
several fissures parallel to the main, uniformly altered pegmatite dyke
and represented by the bedding planes of the schists. They became
active only where they met and intersected the cross-dyke, and felspar
and qlmrtz for replacement thus became available. The phenomena
described also suggests that the cross-dyke, only partially mineralized,
does not go down to ,great depths, and at the time of pneumatolysis
did not stand in direct communication with the body of magma from
which the vapours were derived.
Other examples of the mode of introduction of the metalliferous
gaseous solutions a,re found on Kohero. Wagner5 cites a case of a, wellmineralized body of pegmatite completely pinching out at a, depth of
92 feet. Below this depth the country rock (mica-schist), however,
was finely impregnated with tourmaline and cassiterite along the
continuation of the line of fissure. The metalliferous vapours in this
case produced appreciable alterations only when coming in contact
with the body OIf pegmatite, while below it impregnation of the schists
on a limited scale took place.
More interesting still are features presented by the numerous la,rge
pegmatite dykes on this property. Some of these are very massive and
the majority closely parallel, cutting the strike of the schists with an
angle of 40-45° (Fig. 6). On the whole they are sparsely mineralized.
The interesting feature is, that where these dykes turn into the strike
of the schists, as, e.g., near the dynamite magazine, they are
appreciably greisenized and well mineralized. The same applies to a
few. dykes which, unlike the majority, are paranel tOi the schists along
theIr whole length. A large pegmatitic blow, moreover, occurring on
5
Wagner, P.A.: "The Geology and Mineral Industry of S.W.A.," Geol.
Soc. Mem., No.7, p. 114.
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FIG. 6.
Sketch plan of pegmatites on Kohero.
TIN-BEARING PEGMATITES OF ERONGO AREA, S.-W. AFRICA.
129
this property, is greisenized and mineralized in distinct zonal bands
parallel to the strike of the schists and continuing their bedding planes
across the blow.
At numerous localities the massive dykes, which are not appreciably
mineralized, have sent off narrow offshoots extending into the schists
pa,rallel to their strike or occasiona.lly at right angles to it and sometimes only a few yards in length. Some of these connect closely
adjoining massive dykes. All of these offshoots are heavily greisenized
and mineralized and, in consequence, are being worked for tin.
At
no place were they found adua.lly to cut through the massive dykes,
and a,pparently they represent apophyses of the sa,me magma. The
interesting feature, however, is that the pneumatolytic altera,tion is
carried across the massive pegmatite along the lines of continua,tion of
these narrow offshoots and such sections of the former are being worked
for tin.
It also frequently happens, that there are small wedges of schists
protruding conformable to their strike into the massive dykes, or that
the la,tter are interrupted by such schist wedges. In such cases the
pe,gmatitic rock of the massive dykes is almost invariably also pneumatolytically altered and mineralized on one or both of the contacts
with the schist wedge.
These fea,tures clearly indicate the influence of the bedding planes
of the schists on the direction of pneumatolytic vapours.
Metasomatic process involved.-In this type of tin-bearing pegmatite
the pneumatolytic changes consist mainly of the destructive alteration
of felspar to muscovite and lepidolite, the albitization of. the potassium
felspa,r and the replaeement of felspar and quartz, particularly the
former, by cassiterite and tourmaline. Both the muscovitization and
albitiza,tion of the felspar, as well as its repla,cement by cassiterite,
begin along cleavage cracks.
Pseudo,morphs of cassiteritE) after
felspar aTe very frequently produced in this way. Muscovite and
cassiterite a,re usually directly associated and the heavier the greisenization the richer the tin content. Further, the coarser the muscovite or
lepidolite the larger the crystalline aggregates and lumps of cassiterite.
It is a curious fact that usually both cassiterite and muscovite
avoid large crystals of felspar and very coarse portions of pegmatite
except in their ma,r,ginal portions or along definite clea,vage cracks or
clefts. They are usually far more abundant in the finer-grained
portions of the rock. The easier permeability of finer-grained aggregates
by volatile substances evidently plays an important role in this connection. This feature is not altogether an original one, the finer grain of
the heavily altered portions being in part also due to the destruction of
larger felspa,r crystals by the process of albitization and muscovitization and the production of small grains of secondary quartz.
130
TRANSACTIONS OF THE GEOLOGiCAL SOCIETY OF SOUTH AFRICA.
(c) Intensive greisenization of Pegnw,tite associated with intrusive
bodies of pegmatitic Qua,rtz.
This type is characterized by the intense and frequently complete
greisenization of la.I~ge bodies of pegmatite, often in the form of blows,
along and in the immediate neighbourhood of their contacts with
intrusive bodies of pegmatitic quartz. Usually the replacement of the
felspar by muscovite and its lithia-bearing varieties is most intense
along the quartz contact, and the bands of massive greisen therefore
follow the contours of the quartz bodies. Irregular bands of maSSIve
greisen, 3 to 9 feet in width, and consisting almost entirely of very
,coarse muscovite or lepidolite with spasmodic lumps of cassiterIte
weighing 100 Ibs. or more rB8ult. The greisenization of the pegmatite
in irregular bands ma,y, however, extend for more than 30 feet, away
from the quartz contact.
Not only the pegma,titic blows but also the intrusive body or bodies
of quartz and, as a consequence, also the irregular greisen bands, are
usually arranged pa,rallel to the strike of the schist.
rrhe mineralization of such occurrences is, a.s a rule, highly
erratic. Where the quartz-body does not extend down to any great
depth or rapidly narrows in width, the bands of stanniferous greisen on
either side usuall;y eonverge rapidly, and there is a possibility that with
the actual pinching out of the quartz the greisen band will become
very narrow in width and perhaps limited to the walls of t,he fissure
along which the quartz was intruded. At no place, however, has work
been carried out deep enough to prove this 1 though the converging of
the greisen bands with the attenuation of the quartz can quite clearly
be seen at Sandamab (Fi.~. 7.)
Other occurrences belonging to this type are the " Sidney Mine,"
on Dawib West" and the main working on Drew's claims on Kudubis.
At the former locality black tourmaline (schor!) is developed in great
abundance in distinct zones of alteration, but has very little or no tin
associated with it.
Its zones are also distinct from the greisen bands
and the black tourmaline was evidently deposited during an earlier
phase of pneumatolysis.
The main working on Rubner'sclaims, on Dawib Ost, presents
similar features to this type of tin-occurrence in that lar,cre bodies of
pegmatitic qua,rtz ha,ve been intruded into t,he pegmatite and the
cassiterite appears limited to zones of replacement around the quartz.
Muscovite is here, however, only sparingly developed. Schorl is
abundant, but nOlt in direct association with cassiterite.
A noteworthy feature of all these occurrences characterized by an
intense greisenization is the invariable association of large lumps of
triplite with the greisen and cassiterite. At Drew's working, on
Kudubis, wolframite o~curs with the cassiterite in small quantities.
The most extraordinary instance, of this type js the occurrence on
Irle's claim, on Otjokatjonga and Otjimboyo Ost. Here the body of
J!0gmatite, intruded by lalge masses of pegmatitic quartz, is situated
N
8
6
FIG.
1. Pegmatitic quartz.
7.
Schematic Section through the Sandamap Hill.
2. Stannifuous greisen bands. 3. Pegmatite with mica and tourmaline.
4. Ordinary Pegmatite.
5. Schist.
132
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
in ,crystalline limestones of the ma,rble series and not in schists. '.rhe
pegmatite itself is of the ordinary type, but has suffered extensive
alteration. The initial phas~s of pneumatolysis were marked by the
extensive deposition of black iron-tourmaline (schorl), very frequently
in the form of circular or elliptical rings (Fig. 8), in mode of formation
no doubt somewhat similar to the tourmaline suns and nests so
abundant in the Erongo .granite of the Erongo area. Very frequently
the schorl also encrusts, i.e., repla,ces marginally ]a,rge crystals of
felspar. Whether this early tourmalinization preceded the intrusion of
the pegmatitic quartz it is difficult to say. It must be stated, ho,wever,
that tho,se portions of the pegmatite characterized by schor! are entirely
barren andcaITY no tin.
The next phase of pneuma,tolysis, either contemporaneous with or
Immediately subsequent to the intrusion of the pegmatitic quartz,
brought about the main alteration. of the pegmatite. The cassiterite is
mainly limited to the more intensely altered portions of the rock,
which is characterized by the development of peach-blossom coloured
lepidolite in bands and zones of replacement, particularly around the
margin of quartz bodies. Associated with the lepidolite occur seconda,ry
quartz and a large number of beautiful crystals o,f coloured lithiatourmalines. Their colour varies from dark bottle-green and light seagreen to a delicate pink and wine-red. The bulk of the crystals are
zoned and consist of alternate, green and pink layers.
Beautifully
zoned crystals several inches in diameter are common, but they are
all fractured on a minute scale and on exposure break into small fragments. They are most abundant in the lepidolite-,greisen-bands and
the marginal portions of the pegmatitic quartz; They also occur within
large felspar crystals and are thus obviously formed by the replacement of the latter.
The cassiterite is entirely limited to those portions of thepegmatitic
rock which exhibit an intense replacement by lepidoEte and coloured
lithia-tourmalines during the later pneumatolytic phase. As already
stated, those portions chara,cterized by the earlier-formed schor! are
devoid of cassiterite. The coJoured lithia-tourmalines are frequently
found to be moulded on crystals of cassiterite.
Of special interest is the occurrence of lumps of argentifero'Us
bismuthinite in the lepidolite-greisen-bands, in the marginal portions
of pegmatitic quartz and the contacts of these rocks. It is thus
a,pparently of pneumatolytic origin~ Bismuthite and bismuth Ql~hre occur
as its products of weathering.
About 150 yards away from the main working there occurs a dyke
of massive lepidolite about 2 feet in thickness, but only of short length.
At this locality there is also a small occurrence, of copper sulphides:
bornite with malachite and azurite, a,ppa,rent1y also associated with an
altered pegma.tite. In the crystallme limestones c.£ the same area there
also occur narrow stringers of ga.lena and quartz veinlets carrying
auriferous iron and copper sulphides.
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on Otjakatjongo.
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134
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
Metasomatic process involved.-It appears that originally the
deposits of this type consisted of ordinary pegmatitic rock forming a
blow or irregular thickening of a dyke. Probably the deposition of
schorl during the initial phases of pneumatolysis preceded the intrusion
of the pegma,titic qua,r'tz which was injected as a final magmatic residue
along the same, fissures to form. irre.gula,r bodies within the pegmatite.
The quartz appears to have been heavily charged with volatile constituents, notably water vapour, which diffused into the adjoining
pegmatite and altered it to a considerable extent. :More metalliferous
vapours then ascended along the same fissure and, following the
quartz-pegmatite contact, brought about an intense and, in places,
complete greisenization of the adjoining pegmatite. These processes
a,re simila,f to those of the previous type, restricted to more limited
masses of pegmatite and consequently on a more intense scale. The
cassiterite appears to have- been deposited entirely during the later
pneumatolytic phase and is both contemporaneous with and subsequent
to the intrusion of the pegma,titic quartz. Along' with it, came
muscovite, lepidolite, wolframite, argentiferous bismuthinite, coloured
lithia-tourmalines, triplite and lazulite. The high lithia. and fluorine
content of the associated minerals is worth noting, and also the difference in chemical composition of the tourmalines; the deposition of
schorl at higher temperatures, containing dense radicles of the heavy
metals, and at lower tempera,tures the coloured lithia-tourmalines
with lighter radicles and quantities of fluorine. Volatilized phosphoric
acid appears to have been present in the water vapour responsible for
the bulk of the pneumatolytic alterations.
C.
Cassiterite deposited from Aqu.eous Solution.
This type of tin occurrence is represented by only a few isolated
instances. At Otjimboyo, in the valley of the Hardersrivier, there is
an extensive sheet of pegmatite much jointed and fissured along
tectonic directions. The tin-content of the pegma,tite itself is poor,
but some of the joints contain narrow veins of pure massive cassiterite
bounded on both walls by thin bands of muscovite.
This type shows the presence of a, pneumatolytic mineral, i. e. ,
muscovite, and probably belongs to the last stages of pneumatolysis
and leads over into tin-deposits in which the cassiterite is no longer
associa,ted with pneum'ltolytic minerals, but with minerals cha,raderisti c
of the early hydrothermal pha.s~, and in which its mode of occurrenee
suggests deposition from aqueous solution.
Close to the, former localit,y on Otjimboyo and situated about 10
feet below a pegmatite sheet, occurs an elongated cavity in the schists
about 8 feet in length and with a, maximum height of one foot. When
first opened, the floor, roof and sides were found to be encrusted with
beaut,iful drusy crystals of cassiterite, some of them of large dimensions
and encrusted by pure milky quartz, the latter showing numerous casts
of the former. In addition, films of arsenopyrite were found to encrust
TIN-BEARING PEGMATITES OF ERONGO AREA, S.- W. AFRICA.
135
crystals of cassiterite. Muscovite is still present in limited quantity.
The whole therefore represents a cassiterite druse from which 800 kgs.
of pure cassiterite are said to have been extracted.
Genetically closely related to this oecurrence is a· vein of p'!re
massive cassiterite bounded by a narrow selvage of quartz and occurrmg
in schists on Otjimboyo 10. In its upper portions the. cassiterite vein
was about Ii inches in width, but it gradually thinned out till at a,
depth of 26 feet it pinched out, together with the quartz. In a pro~­
pecting shaft, 177 kgs. of pure cassiterite were recovered from thIS
vein over a vertieal distance of 18 feet.
The largest occurrence of this type is a stanniferous quartz vein
situa,ted on Muller's claims, on Kohero East. Here there is a, quartzreef about 4-5 feet wide at its maximum with a total visible. length of
45 feet. The cassiterite is disseminated throughout the quartz in small
grains and larger crystalline masses and is associa,ted with chalcopyrite
and bornite. Muscovite and tourmaline are entirely absent. Considerable quantities of cassiterite, amounting to a total 0'£ 38 tons, have
been recovered from this quartz-reef during a, pe,riod of four years.
Another similar quartz vein of lesser width occurs near by, but has
not been opened up.
Whne in the case of the other occurrences the cassiterit,e was found
to be a product of pneumatolytic replacement of the felspar and quartz,
in the above examples no replacement of any kind has taken place,
the cassiterite being apparently merely deposited from solutions along
with quartz and sulphides. It seems justified therefore to assign these
occurrences to the early hydrothermal phase, during which cassiterite
was deposited from highly siliceous aqueous solution at a temperature
in the neighbourhood of 358°C.
TilE
GENESIS OF THE CASSITERlTE.
The na.ture of the pegmatites and the mode of occurrence of the
cassiterite having been described in the foregoing pages, it now remains
briefly to discuss its origin and genetic history.
SOUTce of the M etallifetou,s VapouTs and Solutions.
In regard to this point there can be no reasonable doubt. It is
clear that the source of the metalliferous vapours and solutions is the
same as the source of the pegmatites in general, and that the deposits
are connected genetically with the intrusion of the old granites. The
close association of the pegmatites with the various phases of the main
intrusion of old granites, commencing with that of Salem granite, has
already been made sufficiently clear. The tin-deposits are in no way
connected with the intrusion of the very much younger Erongo.granite.
rr'heir occurrence together in the same area is quite accidental.
Excepting a second tourmalinization on a restricted scale, at no place
did these younger granites produce new pneuma,tolytic alterations of
136
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
the old pegmatites. While the age of the Erongo granite is most
probably Karroo or post-Karroo, that of the old granites and associated
pegmatites falls well ba,ck into the Archaean.
'The primary or secondary nature of the Oassiterite as (1 constituent of
the Pegmatites.
Cloos, who was the first to investigate the tin-bea,ring pe.gmatites
of the Erongo area, was of opinion that the cassiterite is a primary
constituent of the pegmatites deposited during their solidification. 6 In
this view he was followed by both Wagner and Reldting (loc. cit.) for
some of the modes of occurrence of the cassiterite, while both recognised the fact that in a number of instances it is clearly of secondary
origin.
Theoretical consideratiolls already suggest that the alteration of
the pegmatitic material by pneuma,tolytic va,pours and .gaseous solutions, thou.gh taking place in several well-marked stages, nevertheless
probably represent's a continuous process. It is quite conceivable that,
since the pegmahtic magma during the process of intrusion no doubt
already contained considerable quantities of volatile constituents,
pneumatolytic action in its strict sense can already set in, while the
pegmatitic magma is solidifying and prior to the complete crystallizationof the main minerals. 'This stage may be referred to 'as the
Primary Pneumatolytic or Pegmatitic phase. Pneumatolytic reactions
taking 'place after the crystallization of the main lllinerals of the
pegmatite and due to the strea.ming up 01£ metalliferous vapours from,
lower, still liquid residual portions of the magma may then be said to
belong to the Secondary or Principal Prieumatolytic phase.
In this way Broegg'er, in his classic researches on the ae.gerite
and nepheline-syenite-pegmatites of southern Norway, 7 has distinguished four periods of crystallization:
(1) The phase of magmatic consolidation, during which the
essential minerals of the pegmatites crystallized and a, subordinate degree of primary, syngenetic pneumatolysis took
place.
' .
",
.
(2) The principal pneumatolysic phase.
(3) The phase of zeolite formation.
(4) The' phase of carbonate and fiuo-carbonate formation.
In general pneumatolytic action may therefore be described as
Syngenetic and Epigenetic.
It has already been shown that with the exception of the quartzcassiterite veins and drusy cavities, the cassiterite in all the tin-bearing
60100s, H.: "Del' }~rongo. Beitrage zur geol., Erforschuncr del' deutch'"
Schntzaebete, Vol. 17. Berlin, 1919, p. 167.
'=',
.,
7 Broegger, W. C.: "Die Mineralien del' Syenitpegmatitgange del' siidnorwegischen Augit- und Nephelin-syenite," Ze'itschr. j. J{rystallograph'ie,
Vol. XVI, 1890.
TIN-BEARING PEGMATITES OF ERONGO AREA, S.-W. AFRICA.
137
pegmatites investigated was found to be a, product of replacement
mainly of the felspar, but also of the quartz constituting the pegmatite. The question now remains: did the replacement of felspar and
quartz by cassite6te commence before or after solidification of the
pegmatite? The evidence available suggests that most probably there
was a considerable interval of time and temperature over which
pneumatolytic alteration took place, and tha,t the- replacement of the
felspar and quartz already commenced before the complete solidification of the pegmatite. Theoret,ieally, of course, this replacement could
commence as soon as crystals of felspar and quartz began separating
out from the magma highly cha,rged with gaseous solutions, but
ma,croscopically and under the microscope it is frequently seen that
the cassiterite avoids the earlier formed coarser aggregates of felspar
and quartz and is most abundant in the finer gra,ined portions forming
a kind of mesostasis between coarser portions. This and other evidence
suggests that at the time of the first appreciable pneumatol;ytic replacement the pegmatite had alr~ady more or less solidified, though it may
still have been in a plastic sta,te.
Although most probably, therefore, .the formation of cassiterite by
the pneumatolytic replacement of felspar and quartz was a long
continuous process extending over a considerable interval of time and
temperature, nevertheless at least two successive stages of replacement may be made out. It is clear, of course, that, their limits are
somewhat ill-defined and tha,t considerable amounts of overlapping
take pla,ce.
CASSITERITE AS THE PRODUCT OF PNEUMATOLYTIC REPLACEMEN'r.
Stage I (Initial phases): Oassiterite occuiting alone
Black Iron-Tourmaline.
01'
associated with
These mode of occurrence of the cassiterite have already been
discussed in detail in the descriptive portion or this paper. It is also
this type of tin-bearing rock of which previous authors considered the
cassiterite to be a prima,ry constituent. It has been shown, however,
that also in these cases the cassiterite is a product of pneumatolytic
replacement and was derived from ascending metalliferous gaseous
solutions. The more uniform and consistent distribution of cassiterite
throughout the rock,i.e., the more general and uniform permea,tion of
the pegmatite by the stanniferous vapours, and the ,greater independence of the cassiterite froni actual fissures and other structnral features,
Dt' well as other evidence, suggest that the deposition of cassiterite alone
or associated with black iron-tourmaline represents the ea.rIier phases of
pneumatolysic alteration of the pegmatites, which took place at a.
higher temperature and proba,bly began before the complete solidification of the pegmatite.
138
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
Stage II (Later phase): Oassiterite accompanied by Greisenization.
Most likely ther~ wa,s no break between this and the former stage,
the different mineral associations of the cassiterite being no doubt due
to the change in composition of the metalliferous vapours with decreasing tempera,ture. During the later phases of pneumatolysis the
pegmatites appear to have been completely solidified, at least in the
horizon now exposed. The metalliferous .gaseous solutions were, now
forced to stream up along narrow fissures and channels such as the
contact walls of pegmatite-dykes, bedding planes of the schists, joint
fissures within pegmatites and schists, etc. While phenomenal enrichments occur locally, the cassiterite deposited during these later stages
of pneumatolysis is far more erratically distributed and very much more
dependent on structural features. Very often it is entirely limited to
the channels of easy ascent for the gaseous solutions.
The cassiterite in the ordinary type of tin-bearing pegmatite
situated in schists, i.e., showing mode-rate amounts of replacement and
an almost constant association of cassiterite with muscovite, probably
belongs to a somewhat earlier stage than the heavily greisenized
occurrences assQiJiated with bodies of pegma,titic quartz. In the latter
case the ascent of the metalliferous vapours was, no doubt, in part at
least, connected with the intrusion of the pegmatite quartz durin,g the
last stage of pneumatolytic action, when the gaseous solutions already
consisted mainly of water-vapour and had become highly silicic.
The Early Hydrothermal Phase.
It is characteristic of all occurrences, in the case of which the
cassiterite was deposited from vapours and gaseous compounds, to find
the cassiterite disseminated as grains or crystalline aggregates throughout the rock. When the temperature, however, has fallen below the
critical temperature of water 358°0., or closely approaches it, so that
the water, if not actually in the liquid phase, is nevertheless in a highly
condensed and less mobile state, one would expect any mineral
precipitated from solution to be deposited in a more or less continuous
manner to form veins or lenticular bodies.
.
This is precisely the mode of occurrence of the cassiterite associated purely with qua,rtz and already described as having been
deposited from aqueous solutions. The temperature at the time of
deposition, if not actually below, must have been at least in the
vicinity of the critical temperature of water, 358°0.
MINERAL ASSOCIATION OF THE OASSITERITE.
Oloos (loc. cit.), led by the interesting phenomenon that in a
n.umber of instanc~s a,. definite band of cassiterite-bearing pegmatites,
sItuated at a defimte dIstance from the parent .granite-massif, is found
to encircle the latter, and that on approa,ching the granite-contact the
cassiterite is often entirely replaced by black iron-tourmaline (Schorl),
promulga,ted the view that schor! a.nd cassiterite always mutually
TIN-BEARING PEGMATITES OF ERONGO AREA, S.- W. AFRICA.
139
exclude each other. In this view he was followed by Wagner (loc cit.).
Very many more occurrences or tin have, however, been discovered
since the visit of these authors to the tin-fields, and very much more
development work has been done since then.
It is clear from all the material collected over the whole of the
tin-fields that the pro::ess of mineralization was not nearly as simple
as imagined by Cloos, but that, on the contrary, it varied very
materially from place to place. There were many factors which may
have influenced the composition of the metalliferous vapours and the
nature of the chemical reactions and minerals deposited from place to
pla.ce. 1'he chief of these seems to have been temperature, and the
temperature gradient in the adjacent sediments away from the parent
magma is found to playa dominant role. Further, when two or more
minerals occur together in the same pegmatite, it does not necessarily
follow that they were deposited at the same time by a contemporaneous
series of chemical reactions. Also pnemnatolytic processes frequently
being of a highly destructive nature, subsequent reactions may completely alter the whole aspect of the mineral associations.
The minerals that have been found occurring together with the
cassiterite, apart from the gangue-minerals, felspar and quartz, and
which, similar to cassiterite, are clearly of secondary origin, are black
iron tourmaline (schorl), coloured lithia-tourmalines, muscovite,
lepidolite and lithia-bearing va.rieties of the former cookeite, etc.,
triplite, lazulite, garnet (a1mandine-spessartite), apatite, topaz,
flu o·rite , beryl, tantalite, wolframite, monazite and columbite,
molybdenite, argentiferous bismuthinite, arsenopyrite, chalcopyrite
and bornite.
The majority of these minerals, however, are only sporadically and
sparsely distributed. Of widespread occurrence and of importance from
a genetic point of view only are black tounnaline and coloured lithiatourmaline, muscovite and lepidolite, triplite, and to a lesser extent the
sulphides.
(a) Black Iron-Tourmaline.
Tourmaline, which is intimately associated with cassiterite in
pegmatites situated in granite (Paukwab and Erongo-Kanona) is
frequently moulded on cassiterite, but the latter is also found to replace
the former. The minerals are probably deposited more or less contemporaneously at high temperatures.
At numerous localities tourmaline is a1so found together with
muscovite and cassiterite in pegmatites situated in schists, but it is
hardly ever in juxtaposition with the latter, and is usually in separate
zones of replacement, and then apparently deposited earlier than
cassiterite.
In many instances black tourmaline and cassiterite mutually
exclude each other, schorl being deposited at higher temperatures in
the parent granite and a belt adjoining the granite conta.et. Further
away from the latter the schod is then replnced by muscovite and
cassiterite, to form a tin-bearing bel/-,.
140
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
It is evident from these brief statements that no hard and fast
rule can be given for the. rela,tionship existing between cassiterite a,nd
black tourmaline. Slight changes in temperature and concentration of
the boron-content in the gaseous solutions a.s against Sn0 2 may bring
about destructive changes. Generally black tourmaline seems to be.
deposited earlier and at a higher temperature than cassiterite. In the
absence of a suitable flux or volatilizer, tin mu.y, however, be deposited
at a higher tempe,rature than usual, contemporaneously with
black tourmaline. It is also obvious that any segment or horizon of al
pegmatite dyke with progressive decline in temperature can successively
come within the sphere of deposition both of black tourmaline. and
cassiterite.
(b) Colou,red Lithia~Tourmalines.
Are very widely' associated with cassiterite and muscovite in schist
pegmatites, and when occurring together with schorl are, always
deposited later than this mineral.
Their association with muscovite and cassiterite and deposition at
a lower temperature than black iron-tourmaline, and their a,bsence in
the high temperature zone, illustrate the. important influence of the
addition of more volatile and the removal of very dense radicles on the
volatility of certain types of compoundS.
Very often they occur
moulded on cassiterite" so that, if anything, they were deposited
slightly la,ter than this mineraI.
(c) Biotite was found to accompany cassiterite only at Paukwab, i.e., in
pegmatites situated in granite. Even in the altered pegmatites this
mineral is very rare, though in the parent granite, e.g., the Salem
granite it is abundant. The reasons for its almost -complete absence
in the pegmatites are probably it,s high freezing point, which is higher
than that 0'£ black iron-tourmaline, and its great instability in the
pres~nce of pneilmatolytic vapours. Its tendency to suffer metasomatic
altera,tions is even greater than tha,t of felspar.
(d) Muscovite and its Lithia-bearing varieties is the. characteristic
mineral associated with cassiterite in schist pegmatites. Microscopic
sections of these rocks show felspar in all stages of repla,cement ranging from incipient to complete. In the incipient stages the felspar is
usually rendered turbid and the' growth of muscovite begins in the.
form of fine, feathery aggregates. In heavily greisenized rocks there is
usually also an abundance of secondary quarlz. rrhe muscovite almost
invariably contains traces of lithium and fluorine and by degrees passes
over into lepidolite. Massive lepidolite occurs associated with cassi~
terite a,t Kohero and Irles claims, near the Goldkuppe.
In microscopic sections the muscovite is frequently moulded on the
cassiterite or fills cra,cks in the latter. The deposition of the minerals,
however, was probably more or less contemporaneo,us, but that of
muscovite continued after the' .gaseous solutions had been exhausted of
their Sn0 2 content.
TIN-BEARING PEGMATITES OF ERONGO AREA, S.-W. AFRICA.
141
(e) T1·iplite.
This fluo-phosphate of iron and manganese is a very regular associate of cassiterite and muscovite in the intensely greisenized types of
tin occurrence. It also occurs, though more sparingly, in the less
highly micaceous pegmatites, as, for instance, on Muller's claim,
Kohero East, a,nd on Elliot's claims below the Hohensteine. At the
latter locality, the triplite was found to enclose icositetrahedra of garnet
(almandine-spessartite), thus showing that the format·ion of the garnet,
here pneumatolytic, preceded that of triplite. Lazulite is associated
with the tin on Dawib Ost and on Gantschow's claims at Tsomtsaub.
(f) Tantaliie and Wolframite.
The tantalite and tungstate of iron and manganese occur
sporadically in the altered pegmatites throughout nearly the whole of
the tin-fields. Their mode of occurrence and mineral associations are
similar to those of cassiterite in the ordinary schist-pegmatite exhibiting a replacement of felspar by muscovite. On Ploeger's claims, near
the Brandberg, there occurs a pegma,titic dyke almost completely
greisenized and studded with small grains and crystals of tantalite, but
containing practically no cassiterite.
The genetic histories of these two
minerals and their temperatures of crystallization are evidently similar
to those of cassiterite, and their association with muscovite and its
lithia- and fluorine-bearing varieties suggests a similar vehicle, i.e.,
water-vapour containing alkaline fluorides in solution.
(g) Sulphides: argentiferous bismuthiniie:
Accompnnying cassiterite, lepidolite and lithia-tourmalines in
greisenized pegmatite intruded by pegmatitic quartz on Ide's claims,
Otjakatjongo.
Ohalcopyrite and bornite: accompanying cassiterite in a dyke of
quartz on Muller's claims, Kohero East.
Arsenopyrite: accompanying cassiterite in a pegmatite, and also in
a cassiterite-quartz druse on Otjimboyo.
METAMORPHISM OF THE TIN-BEARING PEGMATITES.
Owing to the large amount of folding that some of the dykes have
undergone, they have developed a distinct gneissose structure. Some
dykes at Nobgams, rrsomtsaub and the· Von Goldfuss claims have completely lost their original character and ha.ve been changed to
sillimanite-albite-muscovite gneisses.
Thin sections show that
sillimanite, in the form of fibrolite" is one of the last minerals formed,
and it is passing through broken crystals of cassiterite. Associated
with the sillimanite, but previously formed, are large prisms of
142
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
clinozoisite. The formation of the sillimanite at a late stage seems to
indicate that originally it was present in some other form.
'rhe aluminium which these minera.ls require has evidently been
supplied by the parent magma.. Although the granites were observed to
have absorbed considerable amounts of the schists, the pegmatites
have not.
At T'somtsaub and Dawib Ost the presence of the phosphate of
aluminium, lazulite, associated with the cassiterite gives further
evidence of the presence of this radicle in the magma,.
SUMMARY OF THE HISTORY OF THE TIN-DEPOSITS.
PrevailinJ Temperature
Pegmatitic Phase (Phase of Magmatic Consolidation)Crystallization of felspaJ" and quartz to form ")
pegmatite; very subordinate deposition of l About 575°0.
j
biotite.
Principal Pneumatolytic PhaseInitial phases:
Deposition of black iron- } Not far below
toum1aline, cassiterite, garnet and topaz,
5750C.
albitization of potassium felspars.
Later phases: Intrusion of pegmatitic quartz;
continued albitization 0.£ potassium felspars;
deposition of cassiterite, tantalite, wolframite,
coloured lithia-tourmalines, muscovite, lepidolite, secondary quartz, triplite lazulite,
a,plite, beryl, topaz, fluorite, argentiferous
bismuthinite, molybdenite, arsenopyrite.
B~low
575°0.
and
above 358°C.
Early Hydrothermal PhaseDeposition of cassiterite, quartz, arsenopyrite,
chalcopyrite, and bornite, chlorite and seri- } About 358°C.
cite.
Late Hydrothermal PhaseDeposition of Cu- and Fe-sulphides and galena, }
in places auriferous; forma.tion of siderite
Below 358°0.
and chlorite.
Alteration by descending M deonc WaiersFormation of bismuthite and bismuth-ochre, } 0 r din a, r y
malachite and azurite, iron oxides, kaolin
atmospheric
and secondary chlorite.
temperature.
TIN-BEARIKG PEGMATITES OF EROKGO AREA, S.-W. AFRICA.
143
NATURE OF TIlE VEHICLES BY WHICH MINERALS WEI-tE IN'l'RODUCED.
There still exists considerable uncertainty in regard to the nature
of the vehicle or carrier responsibl~ for the introduction of cassiterite.
The intimate association of cassiterite with fluo- and bora-silicates in
many of the classical tin-deposits of the world led to the suggestion
that the tin was most likely introduced in the form of volatile compounds of tin with fluorine and boron. The mineral associations
of the cassiterite in the tin-bearing pegmatites of the Eron,go area
very strongly suggest the active co-operation of those elements and
alkaline fluorides, particularly LiF, as well as of boron compounds.
rrhese mineral associa,tions have already been described. It may also
be noted that Sandberger proved a. small content of Sn02 in many
lepidolites.
It is clear, however, from the description of the greatly varying
mineral associations of the cassiterite in the Erongo tin-fields, t~a,t
most probably no single element or one chemical reaction brought
about the deposition of the cassiterite. The available evidence suggests
that the conditions of deposition varied greatly and that the primary
factor responsible for the different mineral associations and modes of
deposition of the cassiterite was the gradually declining temperature
of the vapours and gaseous solutions and their changing composition
brought about in this way. Several fedures quite clearly prove this.
(a) The Effect of the Ternpcrature Gradient.
Reference has already been made to the fa,ct that in numerous
instances the pegmatites surrounding any parent body of granite
exhibit a zonal distribution of their pneumatolytic minerals: black
iron-tourmaline without cassiterite bein,g practically limited to the
granite body and a zone of pegmatites adjoining the latter, while
cassiterite and muscovite occur in a definite zone of pegmatites further
removed from the parent granite. At Pauk-wab and Erongo-Kanona
cassiterite and black tourmaline without muscovite occur together in
pegmatites situated in granite, and the change to muscovite-cassiteritebearing pegmatites of schist-type is very abrupt and coincident with
the change in country rock The width of the various zones varies
with the width of the strip of schists between two bodies of granite,
and is also dependent on a number of structural features.
T'his common zonal arrangement is no doubt. merely the result of
a temperature ,gradient, the temperature gradually declining in a
direction away from the parent granite-body, both vertically and
laterally. With a progressive decline of t,empera,ture the composition
of the vapours and gaseous solutions on their way from the still
unconsolidated residual portions of the granitic magma will progressively change and different mineral associations will result from
pneumatolytic metasomatic readions.
Reactions proceeding most
favourably and rapidly at higher temperatures will generally be those
involving the less volatile constituents of the gaseous mixture. The
144
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
more volatile constituents will, on the other hand, be able to penetrate
to greater distances before their most favourable rea,ction tempera,tures are reached.
,The freezing points of the new-formed pneumatolytic minerals
naturally have an import.ant bearing in this connection. Black irontourmaline crystallizes sooner than cassiterite and muscovite, and
away from the .granite contact the gaseous solutions will in general
already have been deprived of their boron-content. Minerals formed
by pneumatolytic interaction with the gangue-minerals po.ssessing a
lower freezing point will in general be deposited further a,way from
the granite-contact.
(b) The difference in the mineral associations of Cassiterite in
Pegmatites situated in the parent Granite and in the adjoining
schists.
Temperature is without a doubt also the primary fador in determining this very marked difference previously outlined.
"Granitepegma,tites " a~re characterized by t,he schod-cassiterite combination,
and "schist-pegmatites" by the almost constant association of muscovite and cassiterite. The transition, as already stated, is usually
very abrupt. At Paukwab the actual contact is taken up by a
pegmatite of schist type. This indicates that temperature is not the
only fa,ctor, but that the nature of the country rock also exerts an
influence, though it has already been shown that at Paukwab the
" granite-dykes" belong to a somewhat earlier period of intrusion than
the bulk of the "schist-dykes" exposed on the present surface. A
hi.gher temperature at the time of pneumatolytic altera,tions in the
granite-dykes therefore most read,ily explains this difference, though
not completely. It is suggested that since there is no evidence
indicating any reactions between the country rocks and the
pneumatolytic vapours, except, of course, marginal albitization and
repla,cement of felspar and quartz in the granite-walls of "granitedykes," the very much greater power for absorbing water vapour and
aqueous solution in the gaseous state displayed by the schists, as
opposed to the solid, impervious granite, pla,ys a role, even if a subordinate one. The active mass 0.£ wa,ter in the gaseous state would,
as a consequence, be considerably higher in the case of pegmatites
situated in schists than in the case of dykes situated in solid non-porous
.granite, particularly since the greisenization of the pegmatites usually
commences fro,m the waIls of the dykes. This would in part explain
the abrupt change denoted by the sudden appearance of muscovite in
schist-dykes.
(c) The constant association of Cassiterite and 1\lu8covite.
As previously mentioned, the mineral associations of the cassiterite
strongly suggest the active co-operation of boron compounds and
pa,rticularly of fluorine and alka1in~ fluorides in its introduction. These
TIN-BEARING PEGMATITES OF ERONGO AREA, S.-W. AFRICA.
145
same mineral associations, however, also suggest that the cassiterite
was not deposited as the result of a single chemical action. For how
is the widespread, almost constant association of cassiterite and
muscovite to be expla,ined on this basis, unless it be supposed that the
stannic fluoride, was introduced previous to the water vapour necessary
for its decomposition? It is true that often the cassiterite can be
shown to have been formed somewhat earlier than the bulk of the
muscovite, but, as mentioned above, most probably there was no
appreciable interval of time and temperature between the deposition
of the two minerals, and the formation of muscovite continued after
that of the tin. Further, the fad that almost inva,riably in this type
of occurrence th~ amount of cassiterite present is directly proportional
to the degree of greisenization, and that the coarser the muscovite or
lepidolite the coarser the cassiterite, strongly suggests tha,t the
cassiterite was introduced together with the water-vapour responsible
for the hydration of the polysilicate felspar at high temperature and
pressure to form the orthosilicate m,uscovite.
In the case of the plagioclase, this process also involves the
addition of K 2 0 and removal of Na 2 0. Since, however, albitization on
an extensive scale generally a,ccompanies the muscovitization, both
processes in part merely involve a mutual exchange of alkaIi-radicles.
The frequent occurrence of cassiterite and muscovite in fixed
proportion is so apparent that Reldring was induced to speak of a
muscovite-cassiterite eutectic (loc. cit.). This idea, however, cannot
be upheld, for the minerals are neither primary constituents of the
pegmatite, but products of pneumatolytic replacement of felspar and
quartz. In addition, there aTe extensive sections of greisen and almost
pure muscovite pradicaHy devoid of cassiterite.
This intimate a,ssocia,tion of the two minerals and the frequent
dependance of the bulk of one on that of the other, as well as the
fact that in general they were depe,sited more or less contemporaneously,
strongly suggest that in this type of tin occurrence water-va,pour at
temperatures between 5750C. and 358°C. was the ma.in active carrier
of the tin-oxide. The fact that the muscovite nearly alwa,yshas ft,
slight content of lithia and fluorine and by degrees passes into lepidoIite and is often associated with lithia-tourmalines and occasionally
fluorite, as well as the general acco,mpaniment o,f greisenization by
albitization of the felspars, indicates that the solving power of wa,tervapour was considerably reinforced by the presence of HF or more
probably alkaline fluorides (LiF and NaF) and boron compounds held
in solution.
In the light of this evidence it may safely be assumed, that in
those tin-occurrences in which the cassiterite is associated with
muscovite and lepidolite, the water-vapour responsible for the hydra,tion
of the felspa,rs was also mainly responsible for the introduction of the
cassiterite, its solving power being still further increased by the
presence of alkaline fluorides (LiF, NaF) , . ~oric ~cid and borates, as
well as phosphoric acid (association of triplite and lazulite with
muscovite and cassiterite) held in solution.
146
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
In the case of cassiterite being deposited from aqueous solution
during the early hydrothermal phase, the presence of sulphides, among
them arsenopyrite, suggests tha,t sulphur and arsenic served as additional aids to solution.
In the initial and earlier phases 0'£ pneuma,tolysis, conditions a,ppear
to have been different. In the occurrences belonging to these types
muscovite is sparsely developed or altogether absent, and in the
pegmatites situated in granite cassiterite occurs intimately associated
with bla,ck iron-tourmaline. Hydration of the felspars to form muscovite was quite a subordinate process in the deposition of the tin, and
for this the higher temperature of the vapours and their consequent
different composition were no doubt mainly responsible. But why do
bla,ck iron-tourmaline and cassiterite, in the ordinary case governed
mainly by the effects of a temperature ,gradient, mutuaHy exclude each
other, while in the pegmatites situated in the parent granite at Paukwab
the t,wo minerals are intimately intergrown? Possibly at these higher
tempera,tures, before water-vapour began to take an active part in
the metasomatic rea.etions, boron and tin wer~ originaUy actually
combined. Such compounds of tin and boron exist in nature, as, for
instance, in the rare minerals paigE)ite and hulsite, found in the tinoccurrences of Seward Peninsula, Alaska,. Further, the rare mineral
nordenskioldine, described by Broegger, represents a, borate of calcium
and tin. Subsequent altera,tions of a, related compound of boron and
tin, perhaps by hydrolysis, may then have produced the intimate intergrowth of tourmaline and cassiterite.
In the case of those pegmatites in which the cassiterite is not
associated with any other pneumatolytic rnineral except secondary
albite, it is diffi·cult to express an opinion a.s to the nature of the
carrier of the tin oxide. These occurrences also belong to the initial
phases of pneumatolysis and the cassiterite was deposited at a, higher
temperature. As the cassiterite is a,ccompanied by secondary albite
and bla.ck tourmaline is usually present in small amounts, and moreover compounds containing fluorine and boron are distributed in ,great
abundance throughout the fields, most likely fluorine and boron
compounds were the active agents in these latter cases. It is possible
that the SnOz was deposited by the direct action of SnF 4 on quartz and
felspar, as suggested by Wagner. 8 It is interesting to note in this
connection that the fluo-silicate topaz, which would be one of the
products of the action of SnF 4 on felspars, occurs tUigether with black
iron-tourmaline and cassiterite in a number of localities, particularly
in the pegmatites situated in granite at Pa:ukwab.
Wagner cites as one of the most notable features of the va,rious
occurrences of tin on the Mutue Fides-Stavoren Tinfields in the Transva,aIB the constant and intima,te association of cassiterite with quartz.
In a great majority of cases the cassiterite has selectively replaced the
8 'Vagner, P. A.:
"The Mutue-Fides-Stavoren Tinfields," Geological
Survey Memoir, No. 16, p. 167.
TIN-BEARING PEGMATITES OF ERONGO AREA, S.-W. AFRICA.
147
quartz in preference to felspar. As an explanation, he suggests the fact
that silicon and tin are clos~ly allied elements, both belonging to the
carbon group in the periodic classification, and, even more important,
the fact that quartz and cassiterite possess nearly the same molecular
volume. The replacement of quartz by cassiterite in preference to
felspar would thus involve a minimum expenditure of energy. In the
case of the tin-bearing pegmatites of the Erongo area, just the reverse
is the case. While quartz has also been extensively replaced, nevertheless invariably the cassiterite shows a, preference for felspar. The
same is true of tourmaline, though to a lesser degree, since here the
cassiterite was deposited more or less simult,aneously with muscovite.
While the hydration of the f~lspa,rs was in progress, the stannic oxide
held in solution would also tend to be deposited, replacing the felspar
in process of decomposition by its solvent. The cassiterite would be
most readily precipita,ted where its solvent was actually involved in
chemical reaction with the felspar, particularly since this most
probably also involves a saving of energy, apparently more sal than in
replacing quartz.
A somewha,t similar reasoning would also apply to the association
of cassiterite with tourmaline, which, however, also replaces quartz to
a large extent.
In the case of cassiterite unaccompanied by either muscovite or
tourmaline, microscopic sections show that the cassiterite still preferably replaces felspar, but not to such a great extent as previously, and
for this the fact that the potassiunl felspa,rs were at the same time
undergoing albitization, while the quartz suffered no contemporaneous
change, is probably responsible. It may be, however, that a definite
chemical reaction takes place between felspar and cassiterite or its
fluoric compounds, apart from mere metasomatic replacement. What
this reaction, if existing, actually is, is difficult to say.
Before closing this discussion, it may be pointed out that the
sequence arrived at, i.e., the formation of black iron-tourmaline or
tinstone alon~ o,r in association with each other a,t higher temperatures,
the deposition of muscovite and the bulk of the cassiterite, phosphates
and compounds of lithium and fluorine at lower temperatures and the
precipita,tion of cassiterite from aqueous solution, together with
sulphides of iron, copper and arsenic at lower temperatures still,
closely agr.ees with the composition 0'£ fumaroles with declining
temperature. For it has been found that the hottest fumaroles emanate
essentially compounds of boron, fluorine, chlorine and phosphorus.
With decline of temperature these change into exhalations of sulphur,
arsenic and their compounds, while those possessing the lowest
tempera,ture emit mainly water and carbonic add.
148
'.rRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
VERTICAL RANGES OF THE MINERALS.
Apart fr0'm theoretical consid~rations based on the vertical zonal
distribution of the various minerals found in the Cornish frin-mines
(Malcolm lVlacLaren, Mining Maga,zine, 1917), in the Erongo Tin-fields
there is very little definite evidence to go on. At most localities very
little development work has been done. The deepest shaft put down
does not exceed 240 feet, and that is an isolated instance.
A noteworthy feature of the tin-fields is the occurrence of deposits
belonging to various stages of pneumatolysis and hydrothermal activity
and theoretically to different horizons, side by side on the same level,
i.e., the present day surface. This is no doubt due to the long extent
of the process of granitic intrusion whil~ the ancient sediments were
being actively folded, the intrusion of the magma, in several successive
phases to form numerous isolated and closely adjoining conformable
bodies, the very varying heights within the sedim.ents to which the
various bodies were intruded and the fact that the tin is associated with
more than one phase of the granite intrusion. A considerable amount
of overlapping and intersection of the theoretical horizons is therefore
bound to be the result.
To use the altitudes of the various loca.lities on the present-day
surface to obtain an idea of the thickness of the tin-horizon is not
permissible. It cannot be sufficiently stressed that the present surface
is no indication whatsoever 0'f a, former uniform, h0'rizon, not only on
account of the reasons just stated, but also because subsequent to the
intrusion of the old granites and associated pegmatites the ancient
sediments and intruded rocks have been extensively and intensely
folded, and with them any tin-horizons that ma,y have existed.
Evidence enabling a rough estima,te of the vertical range of
cassiterite in at least one region of the tin-fields is, however, not altogether wanting. Along the western slopes of the Erongo, below the
Hohensteine, some of the occurrences are situated high above those
on the adjoining plains. On Schimansky's claims a deep gully exposes
a measured vertica,l distance between twO' closely adjoining wellmineralized occurrences of about 800 feet. The numerous occurrences
situated at the foot of the mountain on the edge of the adjoining plain
are still 200-300 feet below the lower of these two outcrops. It is
evident, therefore, that at least in this limited region the cassiterite
occurs over a, considerable vertical height.
GeneraUy, however, it must be stated that while there is evidence
of a reliable character,-in the above instance even proof-of the considerable vertical range 0'£ the cassiterites, there is absolutely no
assurance as to the continuity or workability of any particula,r
occurrence in depth, for it is a well-known fact that all tin-occurrences
not of secondary alluvial origin are liable to be erratic in nature.
With the exception of a, few isolated occurrences belonging to the
early hydro,thermal phase, the cassiterite throughout the fields is a,
pneuma,tolytic mineral and was introduced by gaseous solutions that
TIN-BEARING PEGMATITES OF ERONGO AREA, S.-W. AFRICA.
149
travelled along narrow channels under high pressure, and very
frequently appear to ha:ve rushed from point to point, where fa.vourable
conditions existed for their conce.ntration, frequently leaving the intervening spaces absolutely barren. Very seldom did anything like a.
general and uniform permeation of the pegmatitic rock ta.ke place. The
conception of deposition having taken place from gaseous solutions
under higher pressure also serves to explain how very considerable
quantities of cassiterite were frequently introduced from what in many
instances were mere cracks and even submicroscopic channels.